Pulse-compressor system

ABSTRACT

A system for compressing a frequency-modulated pulse signal includes means for separating the signal into a plurality of frequency component signals, a plurality of dispersive media through which the respective signals are passed for compression in a predetermined compression ratio, and means for recombining the compressed component signals into a common output pulse which is a compressed replica of the input pulse.

United States Patent Guilhem et al.

[54] PULSE-COMPRESSOR SYSTEM Inventors: Robert Germain Guilhem, Sceaux;Yvon Pierre Jean Fouch, Chatou,

both of France Assignee: Compagnie Francaise Thonson- Houston Filed:Oct. 30, 1962 Appl. No.: 235,178

US. Cl. ..325/321, 328/56, 333/70 T, 343/ 17.2 PC Int. Cl. ..H04b l/16Field of Search ..343/l7.2; 333/70 T; 328/58, 328/56; 325/321 PrimaryExaminerBenjamin A. Borchelt Assistant Examiner-Richard E. BergerAttorney-Karl F. Ross [57] ABSTRACT A system for compressing afrequency-modulated pulse signal includes means for separating thesignal into a plurality of frequency component signals, a plurality ofdispersive media through which the respective signals are passed forcompression in a predetermined compression ratio, and means forrecombining the compressed component signals into a common output pulsewhich is a compressed replica of the input pulse.

PULSE-COMPRESSOR SYSTEM This invention relates to a system for derivingfrom a frequency-modulated input pulse an output pulse which is acompressed replica of the input pulse.

Since such pulse-compressing operations are of especial utility in radarwork, the invention will be more particularly described with referenceto radar, but this should not be interpreted as limiting the field ofapplicability of the invention.

In the so-called compressed-pulse radar techniques, comparatively longtransmitted pulses are used, in relation to the range-resolving powerdesired, and the received echo pulses are then compressed to thedesired, narrow pulse width. This has the advantage of providing anaccuracy and resolution in range comparable to what is obtainable whenusing narrow transmitted pulses, while at the same time deriving thebenefits of long transmitted pulses operating at high average powerlevels. No narrow pulses characterized by extremely high peak energyneed by transmitted and the attendant pulse-generating difiiculties areavoided. In a compressed-pulse radar system, pulses are generated andtransmitted with a carrier frequency which is modulated linearly withtime about a center value, with a given amount of total variation of thecarrier frequency over the duration of pulse transmission. The receiversystem includes means for imparting to the received echo signal a delaythat is varied linearly with frequency in accordance with a lawcorrelated with the law of frequency modulation applied to thetransmitted pulse, so that a greater delay is imparted to the portion ofthe echo signal corresponding to the start of the transmitted pulse thanto the portion corresponding to the termination of said pulse. The neteffect is to compress the echo signal on the time scale.

In such a system, there are two basic conditions to be observed if it isdesired to obtain a high compression ratio (defined as the ratio oftransmitted-pulse duration to receiver-output-signal duration) withsatisfactory all around operation. They are a high degree of linearityin the modulation of the transmitted pulse with time and a high degreeof linearity in the delay applied to the echo signal with frequency.

It is a specific object of this invention to improve this lattercondition, i.e. improve the linearity of the delay imparted to avariable-frequency pulse, such as a radar echo pulse of the typedescribed, as a function of frequency.

Usually the frequency-dependent delay imparted to a variable-frequencypulse in order to compress it as described above is obtained by passingthe pulse through a suitable dispersive medium introducing a delayvariable with the instantaneous frequency of the pulse. However,considerable difficulties are experienced when it is desired toconstruct such a dispersing medium that will be capable of imparting thedesired, accurately linear variation in delay with frequency, in thecase of broad frequency bands. The broader the band of thefrequency-modulated pulse, the poorer the linearity generally exhibitedby the dispersing media. It is an object of this invention to eliminatethis limitation.

Further objects of the invention include the following: to increaseconsiderably the maximum compression ratio that may be imparted to afrequency-modulated pulse; to facilitate the design and construction ofefficient dispersing units as requiredin the compression offrequency-modulated pulses for radar and related techniques; to providea relatively simple, and highly efficient, pulse-compressing systemhaving excellent frequency response and linearity; to provide improvedradar receiver systems of the compressed-pulse type.

In accordance with a broad aspect of the invention, a system forcompressing a frequency-modulated pulse signal comprises means forseparating the signal into a plurality of frequency component signals, aplurality of dispersive media through which the respective componentsignals are passed to be compressed therein in a predetermined constantcompression ratio, and means for recombining the compressed componentsignals into a common output pulse which is a compressed replica of theinput pulse.

The above and further objects, aspects and features of the inventionwill become apparent as the disclosure proceeds with reference to aspecific exemplary embodiment illustrated in the accompanying drawingwherein:

FIG. 1 is a general block diagram of the improved pulse-compressingsystem;

FIG. 2 shows a frequency response curve of filters used in the system;

FIG. 3 illustrates a corresponding phase-displace ment curve and;

FIGS. 4, 5 and 6 are respective vector diagrams illustrating variousconfigurations of the voltage-amplitude vectors obtained on combiningthe individual frequency-component signals at the output of the system.

In accordance with the invention, a frequency-modulated input pulsesignal such as an echo signal in a compressed-pulse radar system, isseparated into a plurality of component frequency signals applied torespective parallel signal channels; each channel includes a dispersingunit therein which acts to impart to the component signal in therespective channel a compression which owing to the narrow frequencyband of the corresponding component signal will exhibit a high degree ofuniformity. The compressed component signals from the outputs of all thechannels are thereafter recombined, and it will be shown that thisre-combining step introduces an additional, and desirable, degree ofcompression to the signal as a whole. It is known that when compressinga linearly frequency-modulated pulse as used in compressed-pulse radarsystems, by means of a dispersing medium imparting a delay that isvariable with frequency and correlated with the frequency modulation ofthe initial pulse, the echo pulse may in theory be compressed to a finalwidth as small as the reciprocal of the carrier frequency. Inconventional pulse-compressor systems however, owing to the appreciablewidth of the frequency band of the modulated pulse, it has not beenfound possible to provide a dispersing medium capable of imparting thedesired maximum compression with the requisite degree of linearity, sothat the theoretical limit could not, by far, be approached. Theinvention makes it feasible to approach quite closely this theoreticallimit of maximum compression since it breaks down the initial broadbandpulse signal into a plurality of narrow-band component signals, each ofwhich can then easily be handled by a suitable dispersing mediumexhibiting the desired linear frequency characteristics in respect toeach component frequency signal. The invention will now be described indetail with reference to the draw- As shown in FIG. 1, an input signal S(e.g. 3db in strength) is applied simultaneously to the inputs of aplurality of parallel channels identified by the numerals 1 through n.The corresponding component units in all the channels are designated bythe same letter references with a subscript identifying the channel. Theforemost component of each channel is a frequency converter C C C....C,, in which the incoming signal is mixed with a local frequency.According to a preferred aspect of the invention, the local frequenciesapplied to the n frequency converters C through C of the respectivechannels are displaced in accordance with an arithmetic series. Morespecifically, if the input signal S has an instantaneous frequencyvarying in the range from f,, B/2 to f B/2 over the duration T of thetransmitted pulse, then there is used as the first local frequencyapplied to mixer C, a frequency (f B/ 2), as the local frequency appliedto mixer C a frequency f B/2 B/n, and so on with the local frequencyapplied to mixer C being a frequency of f 8/2 (n l) (B/n). As a result,the elementary of component signal delivered into each channel can beshown to have a linear frequency variation from a value (f, f to a value(f,, f B/n over a time interval of T/n. These component signals are thenpassed through respective filters F F F ...F,, each having a pass bandcorresponding to the range of frequency variation of the signal in theassociated channel, as will be evident from FIG. 2, which shows thefrequency response for filter F in the nth channel. The filteredcomponent signals are then applied to respective dispersing units d d,,which serve to compress the signals in a ratio In, in a mannerwell-known per se. It will thus be seen that in each channel, the majorportion of the energy content of the signal is transmitted during a timeinterval T/m l/n.

It is important for the attainment of the foregoing result that thefilters f F shall not introduce any non-linear dispersion of their own.For that purpose it is preferred that the curve of phase displacement Phagainst frequency f within the pass band of each filter should be of thelinear symmetrical shape indicated in FIG. 3 for the nth filter F Thecompressed component frequency signals derived from the dispersing units(1 d are then passed through constant delay networks r r r;,, r whichintroduce into each signal an additional delay independent of frequency,the amount of said additional delay being varied across the channels asa degressive arithmetic series of ratio T/n, being greatest for channelNos. 1 and least for channel No. n. The signals are thereafter appliedto output mixers or frequency converters C C C' C,, in which they aremixed with the same local frequencies as in the input mixers C. Thisensures that the outputs from all the channels have the same frequenciesas at the input to the channels.

Owing to the manner in which the output signals were formed andspecifically to the additional frequency-independent delays imparted tothem in the networks r r it can be shown that all the output signalsfrom the output frequency converters C are coincident during a timeinterval of T/mn starting at the instant (t T t) and terminating at theinstant (t T+ t T/mn where Tis the width of the transmitted pulse, t isa maximum delay introduced by the non-dispersive media traversed by thesignals plus the average delay introduced by the dispersive media, andT/mn represents the duration of the signal in each channel as earlierindicated.

It will be understood therefore that the component signals may all bere-combined at the outputs from the channels to provide a common, usablecompressed output signal. This final re-combining step is performed in aconventional vectorial adder circuit a supplied with the outputs fromall the channels as shown.

It should be noted that the final vectorial addition of the componentsignals will introduce a further com pression, in a ratio equal to n, inadditional to the compression imparted by the respective dispersivemedia. This can be shown as follows. In the final adding or combiningstep, the amplitude vectors of the respective signals are added duringthe selected time interval T/mn, while said signals are being subjectedto a linear phase variation such that substantially at the midpoint ofsaid time interval all the signals are in phase. At that instant theco-phasal component signals can be represented as the series of alignedvectors indicated FIG. 4. Since the mean frequencies of the signals aredisplaced, throughout the interval under consideration, in accordancewith an arithmetic series of ratio B/n, it will be apparent that after acertain time period At, the vector having the highest frequency of theseries has assumed a phase lead of 1r over the vector having the lowestfrequency. The vector diagram will then have assumed substantially theshape indicated in FIG. 5, which shows the signal vectors positioned ona regular polygonal line inscribable in a semi-circle, and substantiallycoincident with such semi-circle provided n is large enough. Since thetotal length of the series chain of vectors must necessarily beconserved unaltered, it will be readily understood that the resultantsignal, in the configuration of FIG. 5, is smaller by a factor of 2/1rthan the maximum value of the resultant signal when all the vectors areco-phasal as in FIG. 4.

Thus, in the condition indicated in FIG. 5, the output amplitude of thesignal has been reduced by an amount of 3db in the example assumedabove. Moreover, some time prior to the instant at which the signals arecophasal as shown in FIG. 4, it will be evident, bearing in mind thatthe phase variations are linear, that there is present a vectorconfiguration symmetrical with respect to that shown in FIG. 5, asindicated by the graph of FIG. 6, when the total amplitude of theresultant signal also is 3db less than that of the sum of co-phasalvectors of FIG. 4. The total duration of the resultant pulse, betweenthe end instants represented by the vector configurations of FIGS. 5 and6 respectively, must then be such as to allow the vector representingthe highest frequency of the series to described one more revolutionthan the vector representing the lowest frequency. Since the frequencydifference is B, the said total duration is 1/8. On the other hand,since the frequency variation in any channel is B/n, the width of thecompressed pulse in any channel is not less than l/(B/n) or n/B. It isevident therefore that the summation of the output signals from the nchannels has introduced an additional compression in the ratio n.

It will be clear from the foregoing that the invention has provided animproved system for compressing radar frequency-modulated pulses,capable of achieving maximum compression ratios in a simple andeffective manner, through the use of a plurality of dispersive unitssimple to design construct and to operate reliably.

While the invention has especial value in connection with radartechniques, it will be easily understood that it may be usefully appliedto any system in the broad field of pulse transmission systems where aconstant compression ratio for frequency-modulated energy pulses may berequired. Further, it will be understood that the system schematicallyshown in FIG. 1 is mainly exemplary, and that various changes may bemade in it without exceeding the scope of the invention, as by omittingor interchanging one or more of the steps shown and/or introducing othersteps, to suit specific applications.

What we claim is:

1. A system for compressing a frequency-modulated input pulse signal,comprising means for separating the input signal into a plurality offrequency component signals", means passing each component signalthrough a related dispersive medium to compress the component signal ina predetermined compression ratio; and means for recombining thecompressed component signals into an output pulse signal which is acompressed replica of the input signal.

2. A system for compressing a frequency-modulated input pulse signal,comprising means defining a plurality of channels having said inputsignal applied in parallel thereto; each channel including a frequencyconverter having the input signal applied to one input thereof and alocal oscillation of predetermined frequency, different for eachchannel, applied to a second input thereof whereby to separate the inputsignal into a plurality of component signals in the respective channels;dispersive means in the respective channels connected to be traversed bysaid component signals to compress the latter in a prescribedcompression ratio; and means for recombining the compressed componentsignals into an output pulse signal which is a compressed replica of theinput pulse signal.

3. The system claimed in claim 2, wherein said local oscillationsapplied to the frequency converters in the respective channels havefrequencies displaced in accordance with an arithmetic series.

4. The system claimed in claim 2, wherein each channel further includesa delay unit connected to the output of the dispersive means forimparting to the compressed component signal an additional delayindependent of the instantaneous frequency of the component signal.

5. A system for compressing a variable-frequency input signal,comprising means defining a plurality of parallel channels eachincluding a frequency converter having the input signal applied to oneinput thereof and having respective local oscillations applied to secondinputs thereof whereby to separate the input signal into a plurality ofcomponent signals in the respective channels; said local oscillationshaving frequencies displaced in accordance with an arithmetic series;dispersing means in the respective channels connected to be traversed bysaid component signals to compress the latter in a prescribed constantratio; delay units in the respective channels for imparting to thecomponent signals additional delays independen of the instantaneousfrequency of said signals, by amounts displaced as between the channelsin accordance with an arithmetic series generally reverse from saidfirst arithmetic series; further frequency converters in the respectivechannels connected for mixing the compressed delayed component signalswith said respective local oscillations so as to restore thereto thefrequency of said input signal; and means for vectorially combining theoutputs from the respective channels into an output signal, therebyintroducing further compression.

6. The system claimed in claim 2, wherein each channel includes aband-pass filter associated with said frequency converter for improvingthe separation of said component signals.

1. A system for compressing a frequency-modulated input pulse signal, comprising means for separating the input signal into a plurality of frequency component signals; means passing each component signal through a related dispersive medium to compress the component signal in a predetermined compression ratio; and means for recombining the compressed component signals into an output pulse signal which is a compressed replica of the input signal.
 2. A system for compressing a frequency-modulated input pulse signal, comprising means defining a plurality of channels having said input signal applied in parallel thereto; each channel including a frequency converter having the input signal applied to one input thereof and a local oscillation of predetermined frequency, different for each channel, applied to a second input thereof whereby to separate the input signal into a plurality of component signals in the respective channels; dispersive means in the respective channels connected to be traversed by said component signals to compress the latter in a prescribed compression ratio; and means for recombining the compressed component signals into an output pulse signal which is a compressed replica of the input pulse signal.
 3. The system claimed in claim 2, wherein said local oscillations applied to the frequency converters in the respective channels have frequencies displaced in accordance with an arithmetic series.
 4. The system claimed in claim 2, wherein each channel further includes a delay unit connected to the output of the dispersive means for imparting to the compressed component signal an additional delay independent of the instantaneous frequency of the component signal.
 5. A system for compressing a variable-frequency input signal, comprising means defining a plurality of parallel channels each including a frequency converter having the input signal applied to one input thereof and having respective local oscillations applied to second inputs thereof whereby to separate the input signal into a plurality of component signals in the respective channels; said local oscillations having frequencies displaced in accordance with an arithmetic series; dispersing means in the respective channels connected to be traversed by said component signals to compress the latter in a prescribed constant ratio; delay units in the respective channels for imparting to the component signals additional delays independent of the instantaneous frequency of said signals, by amounts displaced as between the channels in accordance with an arithmetic series generally reverse from said first arithmetic series; further frequency converters in the respective channels connected for mixing the compressed delayed component signals with said respective local oscillations so as to restore thereto the frequency of said input signal; and means for vectorially combining the outputs from the respective channels into an output signal, thereby introducing further compression.
 6. The system claimed in claim 2, wherein each channel includes a band-pass filter associated with said frequency converter for improving the separation of said component signals. 